The present invention relates to a circuit for muting a pop or a click noise which is generated when power is supplied to or cut off from an audio amplifier.
In an audio system, at the time when the power of the amplifier is turned on or off, a large pop or a click noise is emitted from a speaker. In order to prevent the noise, a muting circuit is provided in some types of audio systems.
FIG. 2 shows an example of conventional audio amplifier circuits, which has an amplifier 1, comparator 2 and a NAND gate 3. The non-inverting terminal of the amplifier 1 is connected to a signal input terminal 4 through a capacitor C1. The inverting terminal of the amplifier 1 is connected to the ground through a capacitor C2 and a resistor R1. The output of the amplifier 1 is connected to the inverting terminal thereof through a feedback resistor R2. The output of the amplifier 1 is connected to a signal output terminal 5 through a capacitor C3 and a resistor R3.
The non-inverting terminal of the amplifier 1 is further connected through a resistor R4 to a stabilized power supply 6 for supplying a constant voltage VREF. Bias voltage Vcc is applied to the input of the stabilized power supply 6 and the amplifier 1. The output of the power supply 6 is applied to one of the input terminals of the comparator 2. The other input terminal is applied with a reference voltage Vc1.
The output terminal of the comparator 2 is connected to one of the input terminals of the NAND gate 3. The other input terminal of the NAND gate 3 is connected to a mute terminal 7 through which a mute release signal is fed. The output terminal of the NAND gate 3 is connected to a base of a transistor Tr1 through a resistor R5. The collector of the transistor Tr1 is connected between the resistor R3 and the output terminal 5. Other resistors R6 to R8 and capacitors C4 and C5 are further provided in the circuit.
When a power switch for supplying the power for the circuit is turned on, the output voltage of the power supply 6 is increased with an increase of the bias voltage Vcc. On the other hand, a change of a bias voltage Vc of the amplifier 1 is delayed from that of the power supply 6 due to the charging current of the capacitors, and reaches the bias voltage Vcc. The output of the power supply 6 reaches the voltage VREF which is about one-half of the Vcc. Furthermore, the bias voltage Vc of the amplifier 1 reduces to the VREF from Vcc. The reducing change of the bias voltage Vc causes a signal to produce the click sound, which is output from the amplifier 1.
However, since a low level output is applied from the mute terminal 7, the level of the output of the NAND gate 3 is high, thereby rendering the transistor Tr1 conductive. Hence the output of the amplifier 1 is conducted to the ground through the transistor Tr1 so that the output of the terminal 5 is muted. Thus, no click sound is emitted from the speaker.
On the other hand, the comparator 2 compares the voltage VREF of the stabilized power supply 6 with the reference voltage Vc1. When the voltage VREF exceeds the reference voltage Vc1, the comparator 2 applied a high level output to the NAND gate 3. Thereafter, the mute release signal is fed to the NAND gate 3 through the mute terminal 7 so that the NAND gate 3 applied a low level output to the base of the transistor Tr1. As a result, the transistor Tr1 is rendered nonconductive, thereby stopping the muting operation.
However, because of the difference between time constants in the amplifier 1 and the stabilized power source 6, the transient response of the bias voltage of the amplifier 1 differs from the transient response of the stabilized power source 6. consequently, the output of the amplifier 1 may be generated after the transistor Tr1 becomes nonconductive so that the click sound may be heard.